The present invention generally relates to a needle for taking up a liquid sample from a closed sample vessel having a self-sealing diaphragm and, in particular, relates to a needle comprised of a capillary tube one end of which has a tapered interior.
German Patent Application No. P 30 37 014 (not pre-published) corresponding to U.S. Patent Application Ser. No. 304,735 filed Sept. 23, 1981 now U.S. Pat. No. 4,393,726 and assigned to the assignee hereof, describes a method for supplying a liquid sample by means of a needle or injection needle having a pointed end. Therein the injection needle is made by laterally bending the end of a capillary and by cutting off this bent-off end at an angle so that the injection needle has a tip substantially on its longitudinal axis and a longitudinal passage turning at the end and opening laterally. The longitudinal passage is of substantially constant cross-section up to its mouth. In taking up a sample into the longitudinal passage of the needle or injection needle, a meniscus of the sample liquid is formed above the outer surface of the needle at the mouth thereof due to the surface tension of the sample liquid. In removing the needle, this meniscus is stripped off by the self-sealing diaphragm. Thereby, the amount of sample being taken is changed in an uncontrolled way and the measurement becomes incorrect. Further, when samples are repeatedly taken from the same sample vessel, the sample remaining on the self-sealing diaphragm can, over a period of time, crystallize due to evaporation of the solvent or contaminate subsequent samples.
Needles are also known in which the capillary tube is closed by means of a conical closing member at its end, the member usually having a lateral aperture for taking up and delivering liquid (leaflet Spitzentyp 5 from the Hamilton Monaduz Company catalogue "Prazisions-Instrumente zur Messung und Dosierung von Flussigkeiten und Gasen"). In this case, however, the strip-off effect occurs as well. Also, some of the sample remains in the sample vessel and cannot be taken up by the needle through the lateral aperture thereof.
Furthermore, another needle described in the above-mentioned printed publication by Hamilton has its tip electrolytically tapered on its outside. Therein, however, the longitudinal passage is a straight passage of continuous constant cross-sectional opening in the end face. With such a needle design, there is the danger that material is punched out from the self-sealing diaphragm and either plugs the needle or crumbles into the sample.